Adoptive cellular therapy of cancer involves ex vivo generation of autologous antigen specific T cells, followed by in vitro expansion and infusion into patients in the hope that transferred T cells will traffic to and eradicate tumor1,2. Recent studies have demonstrated tumor regression and measurable clinical benefit in patients who are otherwise refractory to conventional therapy3,4. One major obstacle to adoptive therapy has been the feasibility of isolating tumor-reactive T cells. T cells recognizing tumor-associated antigens, which are often normal self proteins, exist at very low frequency and exhibit predominantly low target avidity5,6 due to central and peripheral tolerance mechanisms. Moreover, such potentially autoreactive T cells may be suppressed by circulating regulatory cells (Treg), that can lead to impaired proliferative response to antigenic stimulation.
Regulatory T cells (Treg) have been shown to play a critical role in controlling immunologic tolerance to self-antigens as well as presenting a major barrier to the development of effective anti-tumor immunity in animal models7-9. In animal models, tumor antigen specific CD8 cells failed to undergo normal functional maturation in the presence of Treg cells and were rendered incapable of destroying specific tumor targets10. Conversely, depletion of regulatory T cells controlled the growth of melanoma in most mice and promoted long-lasting CD8+ T-cell-dependent protective immunity11, possibly through the recruitment of high-avidity antigen specific CTL12. Recent evidence of elevated Treg cells in the peripheral blood of patients with cancer13-16 and the finding that increased prevalence of tumor-associated Treg in situ as a predictor for reduced survival17 suggest the importance of regulatory control of the endogenous anti-tumor immune response. The suppressive mechanisms at play in vivo18, may also limit the capacity to generate antigen-specific T cells in vitro.
Expression of the forkhead transcription factor, Foxp3, has been linked to the regulatory phenotype. Although Foxp3 is a reliable marker of naturally occurring Treg, as an intracellular protein, it cannot be used as a practical method of sorting for viable Treg cells. Instead, it has been shown that CD4+ cells constitutively expressing CD25hi, are also FoxP3+. We and others have demonstrated that depletion of CD25+ cells in vitro can lead to enhanced generation of CD4+ T cells recognizing tumor-associated self antigens19 presumably by eliminating the inhibitory influence of CD25+ T regs in the PBMC responder population.
IL-21 belongs to the family of gamma-chain receptor cytokines that includes IL-2, IL-7, and IL-15-cytokines that all deliver their intracellular signal through the shared gamma-chain receptor and influence T cell activation and differentiation20-22. Recently, we demonstrated that in vitro exposure to IL-21 (in contrast to other gamma-chain receptor cytokines) can lead to the generation of self antigen-specific CTL in increased numbers and with enhanced avidity and function23. In light of work demonstrating that barriers to optimal T cell development may involve a regulatory component, we postulate that IL-21 may influence the regulatory control of cellular responses to tumor self proteins in vitro.